Method for manufacturing an abradable plate and repairing a turbine shroud

ABSTRACT

The invention relates to a method for manufacturing an abradable plate ( 32 ) for a turbomachine turbine shroud ( 24, 26 ), the method comprising preparing a mixture comprising a cobalt- or nickel-based metal powder and a powder based on a fluxing element, depositing a layer of the powder mixture in a mold, and making the abradable plate ( 32 ) by subjecting the powder mixture layer to a method of SPS sintering. 
     The invention also provides a method of preparing a turbine shroud ( 24, 26 ) for a turbomachine.

BACKGROUND OF THE INVENTION

The present disclosure relates to a method for manufacturing a turbineshroud for a turbomachine.

In numerous rotary machines, it is now known to provide the ring of thestator with abradable tracks facing the tips of the blades of the rotor.Such tracks are made using so-called “abradable” materials, which, whenthey come into contact with rotating blades, become worn more easilythan the blades themselves. This serves to ensure minimum clearancebetween the rotor and the stator, thereby improving the performance ofthe rotary machine, without running the risk of damaging the blades inthe event of them rubbing against the stator. On the contrary, suchrubbing erodes the abradable track, thereby acting automatically tomatch the diameter of the ring of the stator as closely as possible tothe rotor. Thus, such abradable tracks are often installed inturbomachine compressors.

In contrast, use of such tracks is less common in the turbines of suchturbomachines, and in particular in the high pressure turbines in whichphysico-chemical conditions are extreme.

Specifically, the burnt gas coming from the combustion chamber flowsinto the high-pressure turbine at very high levels of temperature andpressure, thereby leading to premature wear of conventional abradabletracks.

Under such circumstances, in order to protect the turbine shroud, it isoften preferred to provide it with a thermal barrier type coating madeof materials that serve to protect the shroud against erosion andcorrosion and that present density that is high, too high for thecoating to be effectively abradable.

Nevertheless, under such circumstances, it can naturally be understoodthat the integrity of the blades is no longer ensured in the event ofcoming into contact with the stator, which makes it necessary to providegreater clearance between the rotor and the stator, and thereforeincreases the rate of leakage past the tips of the blades, thus reducingthe performance of the turbine.

Furthermore, because of spots of rubbing against the blades and becauseof the temperature of the burnt gas, the coating can become damaged,thereby providing the stator with less protection.

OBJECT AND SUMMARY OF THE INVENTION

The present disclosure seeks to remedy these drawbacks, at least inpart.

To this end, the present disclosure relates to a method formanufacturing an abradable plate for a turbomachine turbine shroud, themethod comprising the following steps:

preparing a mixture comprising a cobalt- or nickel-based metal powderand a powder based on a fluxing element;

depositing a layer of the powder mixture in a mold; and

making the abradable plate by subjecting the powder mixture layer to amethod of SPS sintering.

The term “cobalt-based” is used to mean a metal powder in which cobaltpresents the greatest percentage by weight. Likewise, the term“nickel-based” is used to mean a metal powder in which nickel presentsthe greatest percentage by weight. Thus, by way of example, a metalpowder comprising 38% by weight cobalt and 32% by weight nickel isreferred to as a cobalt based powder, since cobalt is the chemicalelement having the greatest percentage by weight in the metal powder.

Cobalt- or nickel-based metal powders are powders that present goodhigh-temperature strength after sintering. They can thus perform the twofunctions of being abradable and of providing a heat shield. Forexample, mention may be made of CoNiCrAlY superalloys. These metalpowders also have the advantage of presenting a chemical compositionthat is similar to the chemical composition of the material forming theturbine shroud, e.g. AM1 or N5 superalloy.

The powder based on a fluxing element makes it possible to reduce thesintering temperature of the powder mixture.

The SPS sintering method (SPS standing for “spark plasma sintering”) isalso known as field assisted sintering technology (FAST), or as flashsintering, and it is a method of sintering during which a powder issubjected simultaneously to high-current pulses and to uniaxial pressurein order to form a sintered material. SPS sintering is generallyperformed under a controlled atmosphere, and it may be assisted by heattreatment.

The duration of SPS sintering is relatively short, and SPS sinteringmakes it possible to select starting powders with relatively fewlimitations. Specifically, SPS sintering makes it possible in particularto sinter, i.e. to densify, materials that are relatively complicated toweld, or indeed impossible to weld, because they are materials thatcrack easily when heated. As a result of selecting SPS sintering and ofthe short duration of such sintering, it becomes possible to make anabradable layer out of a very wide variety of materials.

Furthermore, since SPS sintering is performed under uniaxial pressureexerted by the mold on the powder layer, the shrinkage of the powderlayer that results from the sintering for producing the abradable plateis restricted to the direction in which pressure is applied. Noshrinkage of the powder layer is thus to be observed in directionsperpendicular to the direction in which pressure is applied. Thus, it isrelatively simple to control the dimensions of the abradable plate.

It is possible to deposit at least two layers of the powder mixture inthe mold, the two layers being spaced apart from each other by achemically inert insert.

Is thus possible to make a plurality of abradable plates in a single SPSsintering step. By way of example, it is thus possible to deposit tenlayers of powder mixture, each layer being separated from an adjacentlayer by a chemically inert insert. It is thus possible to form tenabradable plates, each having thickness that may lie in the range 1millimeter (mm) to 5 mm, each of the abradable plates being separatedfrom an adjacent abradable plate by a chemically inert insert.

During SPS sintering, the chemically inert insert makes it possible toreduce chemical reactions between the layers of powder mixtures orindeed to eliminate them.

Since each layer of powder mixture is separated from the adjacent layerby a chemically inert insert, the layers of powder mixture do not sinterto one another and it is therefore easier to make a plurality ofabradable plates that do not stick together.

The chemically inert insert may also be arranged between the powdermixture and the mold.

During SPS sintering, the chemically inert insert makes it possible toreduce chemical reactions between the layer of powder mixture and themold, or even to eliminate them, and thus to reduce any sticking of theabradable plate to portions of the mold, or even to eliminate any suchsticking.

The chemically inert insert also makes it possible to reduce theformation of a layer of carbide at the surface of the abradable platethat is in contact with the mold, or even to eliminate any suchformation. It is desirable to avoid forming such a carbide layer sinceany carbide layer that is formed needs to be removed from the abradableplate before it is used.

The chemically inert insert may comprise boron nitride or corundum.

When the chemically inert insert is said to “comprise” boron nitride,that is used to mean that the insert comprises at least 95% by weightboron nitride. Likewise, when the chemically inert insert is said to“comprise” corundum, that is used to mean that the insert comprises atleast 95% by weight corundum.

The chemically inert insert may be in the form of a layer of boronnitride deposited on the mold by using a spray. The chemically inertinsert may also be in the form of a plate reproducing the shape of theabradable plate. Thus, during the step of SPS sintering, the chemicallyinert insert makes it possible to impart its shape to the abradableplate.

Boron nitride may form an outer layer of the chemically inert insert.

The chemically inert insert may be a plate of dense material covered bya layer of boron nitride deposited onto the plate by means of a spray.

The fluxing element may be silicon or boron.

The powder mixture may comprise a percentage by weight of the fluxingelement that is less than or equal to 5% by weight, preferably less thanor equal to 3% by weight.

The mold may be made of graphite, and the SPS sintering may be performedat a temperature higher than or equal to 800° C., preferably higher thanor equal to 900° C.

The SPS sintering is performed at a pressure higher than or equal to 10megapascals (MPa), preferably higher than or equal to 20 MPa, still morepreferably higher than or equal to 30 MPa.

The mold may be made of tungsten carbide, and the SPS sintering may beperformed at a temperature higher than or equal to 500° C., preferablyhigher than or equal to 600° C.

The SPS sintering may be performed at a pressure higher than or equal to100 MPa, preferably higher than or equal to 200 MPa, still morepreferably higher than or equal to 300 MPa.

The present disclosure also relates to a repair method for repairing aturbomachine turbine shroud, the method comprising the following steps:

removing a damaged abradable coating; and

brazing onto the turbine shroud an abradable plate obtained by theabove-defined method.

The fluxing element included in the powder mixture used for forming theabradable plate also serves to facilitate the method of brazing theabradable plate onto the turbine shroud.

Brazing the abradable plate onto the turbine shroud makes it possible toavoid depositing a new abradable coating directly onto the shroud oronto the shroud sector.

Specifically, after the abradable plate has been brazed onto the turbineshroud, a free surface of the brazed abradable plate may be machined.

An abradable plate that has just been brazed onto the turbine shroud maypresent a free surface that need not necessarily extend the free surfaceof the adjacent undamaged abradable coating. Thus, the free surfaces ofthe abradable plate and of the abradable coating are machined so as topresent a surface for facing the turbine wheel that presents as littlediscontinuity as possible. Specifically, if any such discontinuity ispresent, then the turbine wheel could strike against such adiscontinuity, thereby leading to impacts within the turbine, which isnot desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention appear from thefollowing description of implementations of the invention, given asnonlimiting examples, and with reference to the accompanying figures, inwhich:

FIG. 1 is a diagrammatic longitudinal section view of a turbomachine;

FIG. 2 is a diagrammatic perspective view of a turbine shroud sectorincluding an abradable plate;

FIG. 3 is a diagrammatic perspective view of a stack of abradable platesand of chemically inert inserts;

FIG. 4 is a diagrammatic section view of a stack in the mold for SPSsintering, on a section plane similar to the section plane IV-IV of FIG.3;

FIGS. 5A-5D are scanning electron microscope images showing themicrostructure of the various abradable plates;

FIG. 6 is a diagrammatic view of a shroud sector including a damagedabradable coating; and

FIGS. 7A and 7B are diagrammatic side views of a turbine shroud in whicha portion of the abradable coating has been replaced by an abradableplate, shown respectively before and after machining a free surface ofthe abradable plate.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a bypass jet engine 10 seen in section on a vertical planecontaining its main axis A. From upstream to downstream in the flowdirection of the air stream, the bypass jet engine 10 comprises a fan12, a low-pressure compressor 14, a high-pressure compressor 16, acombustion chamber 18, a high-pressure turbine 20, and a low-pressureturbine 22.

The high-pressure turbine 20 has a plurality of blades 20A that rotatewith the rotor, and vanes 20B that are mounted on the stator. The statorof the turbine 20 has a plurality of stator shrouds 24 arranged facingthe blades 20A of the turbine 20.

As can be seen in FIG. 2, each stator shroud 24 is made up of aplurality of shroud sectors 26. Each shroud sector 26 has an innersurface 28, an outer surface 30, and an abradable plate 32 against whichthe blades 20A of the rotor come into rubbing contact.

The abradable plate 32 is brazed onto the shroud sector 26. Theabradable plate 32 has a free surface 34 and a surface 36 that is to bebrazed onto the shroud sector 26.

By way of example, the shroud sector 26 is made of a cobalt- ornickel-based superalloy, such as the AM1 superalloy or the N5superalloy, and the abradable plate 32 is obtained from a metal powderbased on cobalt or on nickel.

In the described implementation, the shroud 24 is made up of a pluralityof shroud sectors 26 that are assembled to one another in order to forma shroud 24. The shroud 24 could equally well be made as a single piece.

In order to fabricate an abradable plate 32, a mixture is preparedcomprising a cobalt- or nickel-based metal powder and a powder based ona fluxing element. By way of example, the cobalt- or nickel-based powdermay be a powder of the CoNiCrAlY family, and the fluxing element may beboron or silicon. By way of example, the powder mixture may comprise 2%by weight of boron.

As shown in FIGS. 3 and 4, the powder mixture is deposited in the formof layers in an SPS sintering mold 42. By way of example, the mold 42 ismade of graphite. The mold 42 comprises an outer mold 44 forming achamber in which the powder mixture is deposited. The mold 42 also has atop piston 46 and a bottom piston 48 that serve to apply axial pressureon the layers of powder mixture during the SPS sintering step.

FIG. 3 shows a stack 38 comprising two abradable plates 32 with a firstchemically inert insert 40 inserted between them. In this example, asecond chemically inert insert 40 and a third chemically inert insert 41are also arranged on either side of the stack 38 such that each layer ofpowder mixture is sandwiched between two chemically inert inserts 40. Byway of example, the chemically inert inserts 40 may be made from platesof sintered boron nitride.

In the implementation of FIGS. 3 and 4, each abradable plate 32 isobtained by depositing a layer of powder mixture between two chemicallyinert inserts 40 and by performing an SPS sintering step.

FIGS. 3 and 4 show two stacks 38 after SPS sintering, the stacksrespectively comprising two and four abradable plates 32.

Before depositing the powder mixture layer, it is also possible todeposit a layer of boron nitride on the mold 42 by using a spray, inparticular onto the surfaces of the mold 42 that are to come intocontact with the powder mixture layer during SPS sintering. This layerof boron nitride likewise forms a chemically inert insert between thepowder mixture and the mold 42.

The chemically inert inserts 40 may also be made out of a material otherthan boron nitride. The chemically inert inserts 40 may optionally becovered in a layer of boron nitride.

The chemically inert inserts 40, whether in the form of plates or in theform of layers, serve to reduce chemical reactions between the powdermixture layer and the mold 42 during SPS sintering. The chemically inertinserts 40 make it possible in particular to reduce, or even to avoid,any sticking of the powder mixture layer to portions of the mold beforeSPS sintering, and also any sticking of the abradable plate 32 toportions of the mold 42 after SPS sintering.

The chemically inert inserts 40 also make it possible to reduce, or evento avoid, any formation of a layer of carbide on the surface of theabradable plate 32.

It can be understood that the thickness of the abradable plate 32obtained after SPS sintering depends in particular on the thickness ofeach layer of powder mixture deposited in the mold 42, and also on theparameters of SPS sintering. The thickness of the abradable plate 32obtained after SPS sintering may also depend on the grain size and onthe morphology of the powder used. In particular, the morphology of thepowder may depend on the method for manufacturing the powder. Thus, apowder fabricated by gaseous atomization or by a rotating electrode hasgrains of substantially spherical shape, while a powder fabricated byliquid atomization has grains of shape that is less regular.

FIGS. 5A-5D show various microstructures of abradable plates 32presenting respective apparent porosities of about 10%, about 7%, about3%, and practically zero.

It can thus be seen that by modifying the SPS sintering parameters, suchas temperature, pressure, and sintering time, it is possible to obtainabradable plates 32 presenting structures that are different. Forexample, FIG. 7A shows an abradable plate 32 obtained during an SPSsintering step at 925° C. for 10 minutes while applying a pressure of 20MPa. FIG. 7D shows an abradable plate 32 obtained during an SPSsintering step at 950° C. for 30 minutes while applying a pressure of 40MPa.

FIG. 6 is a plan view of a shroud sector 26 including a damagedabradable coating 50. The abradable coating 50 may have been obtained bythe method described above. The abradable coating 50 could also havebeen deposited directly on the shroud sector 26 by using a known method.

In the example of FIG. 6, the abradable coating 50 includes a zone 52 ofdamage due to rubbing, e.g. between a blade and the abradable coating50, and a zone 54 of damage due to thermal degradation of the abradablecoating 50 under the effect of hot gas. In the damaged zones 52, 54, theabradable coating 50 is damaged, i.e. its thickness has been reducedcompared with the original thickness of the abradable coating 50.Nevertheless, in certain circumstances, in the damaged zones, theabradable coating 50 may have been removed completely, so that theshroud 24 is then exposed.

In order to repair the shroud sector 26 having the damaged abradablecoating 50, the abradable coating 50 is removed, e.g. by machining, andthen an abradable plate 32 is brazed, e.g. at 1205° C. in a vacuum, ontothe inner surface 28 of the shroud sector 26.

As shown in FIG. 7A, the shroud sector 26 including a brazed abradableplate 32 is then assembled so as to form the shroud 24. FIG. 7A shows ashroud sector 26 having a brazed abradable plate 32 that is arrangedbetween two shroud sectors 26, each having an abradable coating 50. Oncethe turbine shroud sectors 26 have been assembled together, theabradable plate 32 presents a free surface 34 that need not necessarilyextend the free surfaces 56 of the abradable coatings 50 of the adjacentshroud sectors 26. Thus, the free surfaces 34, 56 of the various shroudsectors 26 are machined so as to present a machined surface 58 that isto face the turbine wheel. As shown in FIG. 7B, the machined surface 58presents as little discontinuity as possible. Specifically, if any suchdiscontinuity is present, then the turbine wheel could strike againstsuch a discontinuity, thereby leading to impacts within the turbine,which is not desirable.

FIGS. 7A and 7B show a single shroud sector 26 having an abradable plate32 brazed thereon. Naturally, a plurality of shroud sectors 26 could berepaired, or indeed all of the shroud sectors 26. The repaired shroudsectors 26 may be adjacent or otherwise.

When the shroud 24 is not divided or divisible into sectors, it ispossible to remove a portion of the abradable coating 50 of the shroudthat corresponds to an abradable plate 32 and then to braze theabradable plate 32 onto the inner surface 28 of the shroud 24. It isalso possible to remove the damaged portion of the abradable coating 50and to cut down an abradable plate 32 or to assemble together aplurality of abradable plates 32 in order to cover the inner surface 28of the shroud that has been laid bare in this way.

The inner surface 28 of the shroud and the blades are once moreprotected effectively by means of an abradable coating 50 and anabradable plate 32 brazed onto the shroud. The shroud 24 is thusrepaired.

Although the present disclosure is described with reference to aspecific implementation, it is clear that various modifications andchanges may be undertaken on those implementations without going beyondthe general ambit of the invention as defined by the claims. Also,individual characteristics of the various implementations mentionedabove may be combined in additional implementations. Consequently, thedescription and the drawings should be considered in a sense that isillustrative rather than restrictive.

1. A method for manufacturing an abradable plate for a turbomachineturbine shroud, the method comprising the following steps: preparing amixture comprising a cobalt- or nickel-based metal powder and a powderbased on a fluxing element; depositing a layer of the powder mixture ina mold; and making the abradable plate by subjecting the powder mixturelayer to a method of SPS sintering; and wherein at least two layers ofthe powder mixture are deposited in the mold, the two layers beingspaced apart from each other by a chemically inert insert.
 2. A methodaccording to claim 1, wherein the chemically inert insert comprisesboron nitride or corundum.
 3. A method according to claim 2, whereinboron nitride forms an outer layer of the chemically inert insert.
 4. Amethod according to claim 1, wherein the fluxing element is silicon orboron.
 5. A method according to claim 1, wherein the powder mixturecomprises a percentage by weight of the fluxing element that is lessthan or equal to 5% by weight.
 6. A method according to claim 1, whereinthe mold is made of graphite, and wherein the SPS sintering is performedat a temperature higher than or equal to 800° C.
 7. A method accordingto claim 1, wherein the mold is made of tungsten carbide, and whereinthe SPS sintering is performed at a temperature higher than or equal to500° C.
 8. A repairing method for repairing a turbine shroud for aturbomachine, the method comprising the following steps: removing adamaged abradable coating; and brazing onto the turbine shroud anabradable plate obtained in accordance with claim
 1. 9. A repairingmethod according to claim 8, wherein after the abradable plate has beenbrazed onto the turbine shroud, a free surface of the brazed abradableplate is machined.